Linear aromatic polyesters prepared from dicarboxylic acids, especially aromatic dicarboxylic acids and bisphenols are well known for their suitability for molding, extrusion, casting, and film-forming applications. For example, U.S. Pat. No. 3,216,970 to Conix, discloses linear aromatic polyesters prepared from isophthalic acid, terephthalic acid, and a bisphenolic compound. Such high molecular weight compositions are known to be useful in the preparation of various films and fibers. Further, these compositions, when molded into useful articles using conventional techniques, provide properties superior to articles molded from other linear polyester compositions. For instance, aromatic polyesters are known to have a variety of useful properties, such as good tensile, impact, and bending strengths, high thermal deformation and thermal decomposition temperatures, resistance to UV irradiation and good electrical properties.
A useful method of forming polyarylates comprises melt polymerization and, more particularly, melt transesterification polymerization of dialkyl or diaryl esters of aromatic carboxylic acids and bisphenols in the presence of a transesterification catalyst. However, polyarylates prepared by melt polymerization are often colored in undesirable yellow to yellow-green hues in their "as prepared" state. This coloration of the polyarylates reduces the commercial value o f the composition for use in molded articles in as much as many end uses of the molded composition or blended formulations require "water white" color. In such instances other colorants must be added to the molding composition in order to mask the yellow color of the polyarylate as formed.
An attempt has been made to improve the color of the melt-prepared polyarylates. Thus, commonly assigned, copending application, U.S. Ser. No. 068,163, filed June 30, 1987, discloses including a cobalt-containing salt in the molten polymerization mixture of monomeric components and polymerizing in the presence of a transesterification catalyst to yield polyarylate which has a blueish tint which is more commercially desirable. Cobalt salts and compounds disclosed as useful in this copending application include cobalt carbonate, cobalt benzoate, cobalt phenate and cobalt acetate. While the process disclosed therein has achieved the goal of improving the color of the polyarylate, improved coloration is an ongoing goal. Improving the color of the melt=prepared polyarylate involves a balancing of complementary colors between the yellow to yellow-green of the as-prepared polymer and the cobalt-containing blueing agent. Polymer color can be objectively measured according to the Hunter scale represented by L, a, and b axes. L is a black-white scale, a is red-green and b is yellow-blue. It is preferred to obtain as close to a neutral color as possible represented by the point 100,0,0 on the Hunter scale. Additionally, the light transmission of the polyarylate must be considered and not adversely affected.
Unfortunately, it has been found that the cobalt salts mentioned above are not very soluble in the monomers, which form the molten polyarylate polymerization mixture, and often precipitate out of the phenol solvent which has been suggested for use in dissolving the salt. This lack of solubility causes the cobalt salt to be present as a dispersion in the monomeric mixture instead of uniformly mixed therein. The final product does not have greatly improved color since the solid or precipitated cobalt salt cannot provide uniform masking of the natural yellow tint of the polyarylate and often contains tiny specks of the cobalt salt as an impurity. Accordingly, there is still a need to improve the color of polyarylates in the "as prepared" state and such need forms the primary objective of the present invention.